1. Field of the Invention
The present invention relates to electronic data transmission apparatus and, more particularly, to an improvement referred to generally as electronic half-tone generating apparatus for enabling a black and white facsimile transmission system to accurately and efficiently transmit and reproduce images including shades of gray as well as black and white.
2. Description of the Prior Art
Although photographic and other images containing gray scale information can very quickly, accurately and efficiently be transmitted using television apparatus, the transmission of such data over band-width limited or otherwise limited transmission media such as telephone lines and the like, is not a simple task since the data must first be modified in some fashion to accommodate the particular medium used.
Many types of machines are known in the prior art which optically scan a document, develop electrical signals corresponding to the content thereof, time compress or otherwise encode the data to improve its transmission speed and format, and then remotely receive the transmitted data and use it to reconstruct a facsimile of the original document. The equipment required to scan documents containing only black and white data and to transmit such information in either analog or digital form to a distant location for reconstruction is relatively simple, and through the use of sophisticated data compression techniques can be made highly efficient. However, where the document includes gray scale information, the techniques used and equipment required become much more complex, and normally the transmission costs go up substantially because of the time required to transmit the data.
For example, some prior art machines send gray scale information by modulating a carrier signal with information obtained by scanning the data in analog fashion. However, analog data is difficult to time compress and in systems where security encoding is required, it is difficult to scramble. Other devices quantize the data into dots and send several digital bits of information corresponding to each of the dots. This approach obviously requires the transmission of a large quantity of data. Still other systems transmit gray scale information by sending data in the form of alphabetical characters or other symbols that are gray scale weighed. However, in order to be efficient the data area covered by a character or symbol must be fairly large and as a result small picture detail is usually lost.
Another technique is to use a half-tone scanner to break up the data into discrete black and white dots and produce an image which can then be scanned to electronically produce digital data for transmission. Although this method is capable of producing a facsimile with very good detail, it requires the use of expensive half-toning equipment and is very time consuming.